When drilling an oil well the drill stem consisting of a plurality of sections of threadedly connected drill pipe sometimes gets wedged against the side wall of the borehole. When this happens a tool commonly known as a drilling jar is used to cause an impact which will "jar" the drill stem and hopefully release the drill stem from its position. The tool is constructed with a hammer portion which upon activation of the tool strikes an anvil portion. The tool is activated by a predetermined plateau for tension if it is desired to jar up or compression if it is desired to jar down. The tool telescopes until the hammer and anvil portions strike with a jarring impact. It is common in the art for the drilling jars to use hydraulic release mechanisms. Hydraulic release mechanisms can be of varying designs, but usually have a primary fluid passage which is obstructed by a valve positioned in a restrictive bore. The valve configuration prevents the free movement of the hammer and anvil portions until such time as the valve moves out of the restrictive bore. In order to effect movement of the valve, hydraulic fluid slowly bleeds through a fluid bypass creating a time delay until the valve clears the primary fluid passage allowing free movement of the hammer portion and anvil portion of the tool. When the restrictive bore is no longer obstructed by the valve, the hammer and anvil can telescope unobstructed to create the desired impact.
At the present time most hydraulic drilling jars are only capable of jarring in one direction. Those hydraulic drilling jars which are two way jars have two separate activating mechanisms which artificially lengthen the tool and result in unnecessarily complex valving.